The present invention relates to a thermal energy storage system and particularly to a latent and sensible heat storage and transfer system useful for heating, air conditioning and process cooling.
Rising electrical utility production costs and increased demands for power during peak periods have led to increasing costs to utility companies and consumers. Electrical load management has become important, especially during peak demand periods, such as the air conditioning months of summer. Utility companies, in an effort to reduce costs, have begun offering incentives including reduced rates and subsidies for off-peak usage.
Thermal energy storage systems have been proposed as a means to shift power consumption from peak demand periods to off peak periods. For example, it has been proposed to incorporate a cool storage medium in a heat pump or air conditioning system, with the medium being cooled during off peak hours and then used to cool an enclosed space (e.g., a room) during peak hours. Typical state of the art thermal energy storage systems include solar systems and phase change material or latent and sensible heat storage systems. The latter systems, such as ice storage systems, store heat, or in effect, cold, as sensible heat and as the latent heat of a phase change. Conventional chilled water systems are thermal storage systems that store sensible heat. Unfortunately, all of these systems require large volumes per unit of heat exchange, and generally are not very efficient because of low volumetric heat capacities. Ice storage systems additionally require a cooling unit to operate below the freezing temperature of water since they use the heat capacity of the ice/water transition.
The use of phase change materials offers advantages over sensible heat storage systems. Phase change materials such as ice, hydrated salts, and gas hydrates require smaller volumes to store and to release a given amount of thermal energy because their high latent heats.
Standard heating, ventilating, and air conditioning (HVAC) practice requires air conditioning delivery temperatures or 5.degree. C. to 13.degree. C. Ice storage systems are currently being incorporated in some newly constructed buildings, but remain somewhat deficient in performance. Although ice storage systems have high latent heat capacities, ice that is formed on the evaporator surfaces acts as an insulator. This reduces the heat transfer coefficients and thermodynamic efficiencies.
Extensive research has been conducted into hydrated salt systems. The only energy that can be retrieved reliably is that energy released by crystallization of the aqueous phase in a salt solution. For this type of system to be feasible, large storage tanks are required to increase the heat transfer surface area and improve the heat transfer rate. Such large storage tanks would however, increase the amount of ambient heating, which is inefficient, and therefore would decrease overall performance.
Some research has also been done with gas hydrate systems, but their potential has not yet been developed. Gas hydrate thermal energy storage systems utilize the properties of water to improve heat transfer coefficients and thermodynamic efficiencies.
Moreover, there is concern about the use of many refrigerant gases, particularly those Freon.RTM. and related gases that are considered to be ozone layer hostile. This search has lead to newer, less harmful refrigerants to replace Refrigerants 12 (CCl.sub.2 F.sub.2) and 22 (CHClF.sub.2). Thus, the use of some of these newer, less harmful refrigerants in gas hydrate thermal energy storage systems promises a sensible solution to the energy conservation and environmental problems that currently confront the world.